1. Field of the Invention
The present invention relates to a dental composition and an artificial tooth with the use of said dental composition, In more detail, the invention relates to a dental composition which has good mechanical properties and mechanical machinability, which is superior in color stability, coloring resistance and aesthetic merit, and which is suitable for an artificial tooth, as well as to an artificial tooth by the use of said dental composition.
2. Description of the Prior Art
An artificial tooth is generally required to satisfy various requirements, for example, good mechanical properties such as wear resistance, strength, and impact resistance; good mechanical machinability; good color stability and coloring resistances; superior aesthetic merit; absence of biological detrimentality; and a good biological compatibility etc.
As artificial teeth, resin teeth comprising polymethylmethacrylate as a main component, hard resin teeth comprising multifunctional methylmethacrylate and inorganic fillers, as well as porcelain teeth have been developed and are in use.
As resin teeth, those comprising methacrylate polymers and copolymers mainly consisting of methacrylate have been developed and are in use. Although those are cheap and superior in aesthetic merit, color stability and impact resistance, they have low and insufficient wear resistance. Since resin teeth are exposed to a high temperature when ground with abrasives such as carborundum point or stamp bur for occlusal adjustment by means of a laboratory machine etc., resins are molten and adhered to the abrasives, which lowers grinding efficiency extremely. Further, even though there are some differences in occlusive force during mastication of food between different persons, the force is said to correspond generally to the weight of the person. Thus, when such occlusive force being subjected to a tip of a tooth, occlusive pressure at the tip of the tooth caused by the occlusive force becomes vast. From the above-mentioned causes, there may be easily formed wear and damage in resin teeth due to occlusion during mastication or use of a tooth brush etc. Further, tooth shape may be changed by strong mastication, thus it is difficult to use it stably for a long period of time.
With a view to overcoming the defects of resin teeth; hard resin teeth have been developed Those are composite materials (composite resins) in which polymerized and hardened resins are used as matrices. The resins are prepared by kneading and dispersing inorganic fillers into multifunctional methacrylate monomers and/or oligomers. The hard resin teeth have an aesthetic merit necessary for anterior teeth and sufficient mechanical strength against occlusive pressure for molars. Further, they have preferably improved wear resistance since the inorganic fillers have high hardness. However, the inorganic fillers dispersed in the matrix resins are dug up and exposed at the surface by form-correction or polishing, or are peeled off the surface. Thus, the surface of the hard resin tooth becomes rough, resulting in deteriorated touch with the tongue. Further, dental plaque or dental calculus is deposited in voids formed by peeling of the inorganic fillers from the hard resin tooth. Thereby, the hard resin tooth may be colored. Since the multifunctional methacrylate polymer itself has poor color stability, it may be discolored during use in the mouth. Additionally, since the hard resin tooth has higher hardness than the conventional resin tooth, fractures or chips may be formed during laboratory procedures and a cusp may be easily fractured by strong mastication in the mouth.
A porcelain tooth, one kind of ceramic artificial teeth used for a long time, is high in wear resistance, excellent in aesthetic merit, color stability and coloring resistance, free from biological detrimentality or irritation in the oral condition, good in touch with the tongue, and is a stable dental material exhibiting no change in material quality even in long-term use. However, as compared with resin and hard resin teeth, it is defective in that it is high in cost, poor in elasticity, low in impact resistance because of brittleness, and tends to suffer cracking and fracture. Further, there occurs a considerable shrinkage caused by firing, thus making it difficult to accurately sinter into a prescribed shape. It is the usual practice to use a porcelain tooth after firing, polishing and form-correction. Thermal stress during correction polishing causes microcracks. And those microcracks grow larger through repetition of occlusive pressure during mastication of food and impact caused by chewing hard food, thus resulting in fatigue breakage, or under an excessive occlusion load, in breakage. When a porcelain tooth is used, mechanical retention means such as a pin or a retaining hole is provided in the porcelain tooth to achieve an integral connection of the porcelain material and the denture base resin, because of a very low adhesion between the porcelain tooth and the denture base resin serving as a denture or a support. Such retention means tends however to be subjected to stress concentration, and a defective attachment causes cracks in the surrounding denture base resin or breakage of the porcelain tooth after attachment in the mouth. The porcelain tooth may also tend to come off the denture base resin. For these reasons, manufacture of a satisfactory denture using a porcelain tooth requires skill and a high cost.